The 3rd Generation Partnership Project (3GPP) has started to work on the standardization for the fifth generation mobile communication system (5G), i.e., 3GPP Release 14, in 2016 to make 5G a commercial reality in or after 2020. 5G is expected to be realized by a combination of continuous enhancement/evolution of LTE and LTE-Advanced and an innovative enhancement/evolution by an introduction of a new 5G air-interface (i.e., a new Radio Access Technology (RAT)). The new RAT supports, for example, frequency bands higher than the frequency bands (e.g., 6 GHz or lower) supported by the continuous evolution of LTE/LTE-Advanced. For example, the new RAT supports centimeter-wave bands (10 GHz or higher) and millimeter-wave bands (30 GHz or higher).
In this specification, the fifth generation mobile communication system is also referred to as a Next Generation (NextGen) System (NG System). A new RAT for the NG System is referred to as a New Radio (NR), 5G RAT, or NG RAT. A new radio access network (RAN) and a core network for the NG system are referred to as a NextGen RAN (NG RAN) or a New RAN, and a NextGen Core (NG Core), respectively. A radio terminal (User Equipment (UE)) capable of being connected to the NG System is referred to as a NextGen UE (NG UE). A base station that supports the New Radio (NR) is referred to as an NG NodeB (NG NB), an NR NodeB (NR NB), or a gNB. The names of the RAT, UE, radio access network, core network, network entities (nodes), protocol layers and the like for the NG System will be determined in the future as standardization work progresses.
Further, the term “LTE” used in this specification includes enhancement/evolution of LTE and LTE-Advanced to provide interworking with the NG System, unless otherwise indicated. Such enhancement/evolution of LTE and LTE-Advanced for interworking with the NG System is also referred to as LTE-Advanced Pro, LTE+, or enhanced LTE (eLTE). Further, the terms related to LTE networks and logical entities used in this specification, such as “Evolved Packet Core (EPC)”, “Mobility Management Entity (MME)”, “Serving Gateway (S-GW)”, and “Packet Data Network (PDN) Gateway (P-GW)” include these enhancement/evolution to provide interworking with the NG System, unless otherwise indicated. The enhanced EPC, MME, S-GW, and P-GW are also referred to as, for example, enhanced EPC (eEPC), enhanced MME (eMME), enhanced S-GW (eS-GW), and enhanced P-GW (eP-GW).
As described above, the NG system supports higher frequency bands (e.g., 6 GHz or higher). In order to provide required coverage on these higher frequency bands, higher antenna gain is required to compensate for path loss. Meanwhile, since the size of an antenna element becomes smaller as the wavelength becomes higher, a multi-antenna system using an extremely large number of antenna elements (e.g., several hundreds of antenna elements) can be implemented in a practical size in higher frequency bands. Accordingly, in the NG System, larger antenna arrays are used to form high gain beams. The beam means a radiation pattern at least having several levels of directivity. High gain beams are narrower than wide sector beams used in lower frequency bands (e.g., current LTE bands (6 GHz or lower)). Accordingly, multiple beams are needed to cover a required cell area.
NR NB may use a plurality of Transmission and Reception Points (TRPs). A TRP means a physical location for transmission and reception of radio signals. TRPs may be arranged in a centralized manner or in a distributed manner. Each TRP may form multiple beams. The TRP may also be referred to as a remote radio head (RRH).
Several proposals regarding beam-related procedures in the NR system have been made (e.g., see Non-Patent Literature 1-6). The beam-related procedures include mobility (procedure) and beam management (procedure) (Non-Patent Literature 6). The beam management is a set of layer 1 (L1)/layer 2 (L2) procedures to acquire and maintain TRP(s) and/or UE beams that can be used for downlink (DL) and uplink (UL) transmission/reception. The beam management at least includes beam determination, beam measurement, beam reporting, and beam sweeping. The beam determination is a procedure for TRP(s) or UE to select its own transmission (Tx)/reception (Rx) beam(s). The beam measurement is a procedure for TRP(s) or UE to measure characteristics of received beamformed signals. The beam reporting is a procedure for UE to report information regarding beamformed signal(s) based on the beam measurement. The beam sweeping is an operation for covering a spatial area with beams transmitted and/or received during a time interval in a predetermined way.
Further, it is assumed that the NR uses different sets of radio parameters for a plurality of frequency bands. For example, subcarrier spacing, symbol length, Transmission Time Interval (TTI), and subframe duration are different for frequency bands. These set of radio parameters are referred to as numerologies. In the NG system, the UE and the NR NB support aggregation of multiple NR carriers with different numerologies. The 3GPP has studied achievement of the aggregation of multiple NR carriers with different numerologies by lower layer aggregation, such as existing LTE Carrier Aggregation (CA), or higher layer aggregation, such as existing Dual Connectivity (e.g., see Non-Patent Literature 7-9).